1. Field of the Invention
The present invention relates to a method of compensating a failing nozzle of a print head of an inkjet printer, the inkjet printer comprising at least one print head, the at least one print head comprising a plurality of nozzles. A receiving material is moved relative to the at least one print head. The method comprises the steps of ejecting droplets of marking material from the plurality of nozzles onto the receiving material forming dots of an image, scanning the printed dots, and analyzing the scanned dots for detecting whether a nozzle is failing.
2. Description of Background Art
In inkjet printing, nozzle failures may be caused by nozzle clogging, contamination of a plate in which the nozzles are formed, events in which the nozzles are touched by the receiving material, misdirection of marking material from a nozzle and the like. Such nozzle failures are a serious threat to reliable ink jet printing and to print quality. Therefore it is necessary to avoid a nozzle failure and to detect a nozzle failure as soon as possible after the moment in time of failure of the nozzle.
In a single pass print process, the print head and the receiving material are moved relative to one another in such a manner that each location on the receiving material is exposed to the nozzles of the print head only once. When the width of the print head is at least as large as the width of the receiving material, the receiving material may be moved past the print head in a uniform direction, or, conversely, the print head may be moved over the receiving material only once. When the print head does not cover the entire width of the receiving material, it is moved in a main scanning direction across the paper so as to print one or more lines, and the paper is then advanced in a sub-scanning direction, so that another swath of the image will be printed in the next pass of the print head. Such a single pass process is particularly vulnerable to nozzle failures because there are only limited possibilities to compensate nozzle failures by printing extra dots with other, still intact nozzles of the print head.
Another approach to improve reliability in ink jet printing involves an automatic nozzle failure detection, which permits taking measures for removing the nozzle failure before a larger number of defective images are printed. For example, nozzle failure may be detected by printing a test pattern and then inspecting the test pattern from time to time. However, this method causes a waste in paper and marking material, especially when the test is repeated in short intervals. Moreover, this method requires a sheet disposal trajectory in the paper pass of the printer, so that the sheets carrying the test pattern may be disposed.
Another method of nozzle failure detection involves inspecting the image that has been printed in accordance with the print data. This is advantageous since a nozzle failure can be detected immediately, and the running print process may be stopped, if necessary. However, depending on the nature of the print data, it may be difficult to detect a specific nozzle that is failing from a scanned print.
Methods have been developed to identify failing nozzles using printed information. Such detection, which uses printed information, tries to identify failing nozzles by means of detecting certain stripes in the prints, by scanning all the prints on the fly with a high speed scanner. A problem which arises with such detection is that a nozzle failure can be detected, but not the exact nozzle number that has failed. Faults may be caused by, for instance, misalignment of the print heads, local nozzle side shooters, and scanner artefacts (aberrations). A scanner may be positioned anywhere along the print path for scanning prints that have been printed by the print head. Variations over time, like different coefficients of expansion, suction belt oscillations, nozzle side shooter variations, etc. make the allotment of a certain stripe to a specific nozzle number not possible with high certainty. Such a method is described in U.S. Application Publication No. 2013/0222455. A disadvantage of this method is that multiple incorrect images with serious print artefacts are printed until the exact nozzle number is identified and a correct nozzle compensation can be applied.
By various calibrations, the failing nozzle may be denoted within a few different nozzle numbers, but not the exact nozzle number. This is a problem, because if nozzle failure correction is used, the exact nozzle failure position must be known, otherwise the correction could cause the nozzle failure stripe to be worsened.